In the membrane lecithins and phosphatidylethanolamines of vertebrates there occur large proportions of fatty acids of the linolenate family, especially docosahexaenoic acid. As yet, the functions of such docosahexaenoate lipids remain unknown. To test the function of n-3 fatty acids (lineleate family), in vivo, rats will be made deficient in these fatty acids by means of a linolenate-deficient diet. This will be semipurified, adequate diet in which the only source of essential fatty acid is pure methyl linoleate (1.25 percent by weight). The control diet will be identical except that it will contain 1.0 percent methyl linolenate. From previous experience we know that development of this deficiency will be very slow, so that we will need animals of the second and even third generation. We will measure growth and organ weights, and investigate tissue lipid and fatty acid composition in detail. We also plan to study visual and muscular function in vivo. We will also investigate the formation of lecithins and phosphatidylethanolamines in vitro, using a rat liver microsomal system. These experiments should show the influence of fatty acid structure upon the rates of formation of lecithins and phosphatidylethanolamines.